In the case of a traditional sliding gate valve as described in Swiss Pat. No. 455,169, the slide housing which accommodates the stationary bedstone and has gliding guide elements for the movable slide part is firmly screwed to the bottom of the ladle. This means that disassembly of that gate valve, that is to say the removal of the movable slide part as well as removal of the stationary bedstone can be accomplished only while the gate valve remains secured on the ladle. Likewise the insertion of a new bedstone, the assembly of the slide and the tightening of the clamping screws can also be accomplished only on the ladle. For such operations, it is necessary that the ladle be disposed horizontally. For the insertion of the bedstone, a mounting device is often available, the handling of which however requires a high degree of care in order to assure that the bore of the bedstone and the bore of the slide stone in the assembled state of the gate valve and in its opened position are exactly in alignment. The assembly itself requires a neat method of operation. Also the proper tightening of the clamping screws is essential for the operational safety of the gate valve and requires sensitivity and reliability.
All these requirements are hardly compatible with the rough operation in the steel mill or in the foundry and with the capabilities of the majority of the personnel employed there. Again and again breakdowns with serious consequences occur in the operation of the gate valves because work which is of importance for its safety of operation has to be carried out at different work stations.
Recently, gate valves have become known, which form a self-contained unit and which contain all the refractory and mechanical parts including the slide housing, the movable slide part, hold down plate or lid and clamping screws as well as the coupling elements needed for a connection of the slide part with the drive. Such a unit is releasably connected with the casting ladle by means of bolts and wedges. The advantages of these unit type gate valves lies in the fact, that the overhauling, that is to say the clearing of the worn-out refractory material, the insertion of the new refractory parts and the reassembly need no longer take place in the rough conditions of the plant foundry room but in a central preparing room, reserved for this purpose. Furthermore one must consider it to be of advantage that the idle time of the ladle becomes essentially shorter as a result of the quick replacement of the unit. As compared to that, nothing has changed in the work expenditure for the renewal of the refractory parts.